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LS: Speaking of Science Column December 01, 2008

Feeling the pressure? It’s perfectly natural

The physical phenomenon we call “pressure” affects many of the things we take for granted in our everyday lives. This article will touch on just some of them.

Pressure is mathematically expressed as force divided by area. Think of this equation when you consider a person walking in the snow. The person’s weight is not the sole determining factor of whether the person sinks. Rather, it is the pressure the person’s foot makes on the surface of the snow.

A heavy person using large snowshoes might not sink as far as a much lighter person wearing smaller snowshoes.

The next time you have a vaccination or donate blood, consider this: The difference between a sharp needle and a blunt one is the amount of force the nurse must exert to get the needle to penetrate.

The sharp one has less area over which the force must be distributed, thus resulting in higher pressure for a given amount of force.

Inflation by a pump is something we’ve all experienced. Forcing more air into our bicycle or
car tires keeps them properly inflated for easier rolling.

We also blow air into balloons to inflate them, but not without limit. The enlargement is resisted by the tension in the surface of the balloon and, most importantly, by the external pressure that surrounds the balloon.

The less the pressure pushing on the outside of the balloon, the greater the expansion of the balloon. This is regularly demonstrated at the Western Colorado Math & Science Center with balloons and marshmallows in a vacuum container.

That external pressure comes from what we refer to as atmospheric pressure. It’s the weight of the air above us (yes, air is not weightless) that creates the pressure pushing against the balloon.

In the Grand Junction area, that pressure is on the order of 12 pounds per square inch; that is, the equivalent of the weight of a gallon and a half of milk pushing on every square inch of the balloon.

That might seem quite surprising, but it is the case.

We live in an ocean of air, and the deeper we are in this ocean, the more air there is above us, and the weight of a column of air in a one-inch-square tube is naturally going to be heavier.

At sea level that column will weigh 14.7 pounds; on Mount Everest that column will weigh only 4.3 pounds. We don’t feel the pressure because it acts upon us equally in all directions, but the pressure is still there.

Because the weight of the air above Vail Pass is less than that in Grand Junction, the external pressure is less.

This affects that bag of potato chips you bought on your way home from Denver. You’ve probably observed your potato chip bag becoming firmly inflated under those conditions. It’s been reported that a trucker carrying his potato chip cargo over the pass had to make a visit to the landfill because each bag had split open during the transit, probably because of a defect in how the bags were sealed.

Atmospheric pressure acting on the surface of your glass of soda is what makes the straw an effective means of getting the soda into your mouth. When you suck air out of your straw, the pressure inside the straw above the liquid is reduced, and the atmospheric pressure on the surface of the soda pushes the soda up the straw into your mouth.

As you can see, pressure is an important aspect of our life. There are many other types of pressure that explain the world we live in, but these will have to be the subject of another article.

So the next time someone asks you if you are under pressure, you can say, “Yes, of course I am.”

Allan Conrad is a retired project manager for the Jet Propulsion Laboratory, Pasadena, Calif. He has volunteered for eight years at the Western Colorado Math & Science Center and is a a math tutor at Grand Junction High School.

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